By Lukeron Lubricants · Last updated
Quick answer
Compressor oil grade is the ISO VG number: how thick the oil is at 40 °C. Rotary screw and vane compressors usually use ISO VG 46 or 68. Reciprocating (piston) compressors usually use ISO VG 100, and ISO VG 150 for units with higher discharge temperatures. A higher number means a thicker oil, not a better one. The grade in the compressor maker's manual always comes first.
Compressor oil grade chart
| ISO VG grade | Viscosity at 40 °C (cSt) | Oil is | Typically used for |
|---|---|---|---|
| VG 46 | 41.4 to 50.6 | Thinner | Rotary screw and vane compressors at normal temperatures |
| VG 68 | 61.2 to 74.8 | Medium | Screw and vane compressors in hot rooms or long duty, or where the maker asks for it |
| VG 100 | 90 to 110 | Thicker | Reciprocating (piston) air compressors, usually single-stage |
| VG 150 | 135 to 165 | Thickest | Reciprocating compressors with higher discharge temperatures, such as many two-stage units |
Viscosity limits are the ISO VG classes (ISO 3448, plus or minus 10% of the middle value). The uses shown are typical. Follow the compressor maker's manual.
Compressor oil comes in a few common grades: ISO VG 46, 68, 100 and 150. Which one is right depends mostly on the type of compressor. Screw compressors usually run a thinner oil, and reciprocating (piston) compressors usually run a thicker one. The wrong grade does not stop the compressor on day one. It shows up later as heat, oil carried into the air, carbon on the valves and a worn machine. This guide explains what the grade means and how to choose it.
What does compressor oil grade mean?
The grade is the ISO VG number, from the ISO viscosity grade system. It is how thick the oil is at 40 °C, measured in centistokes (cSt). VG 46 is about 46 cSt, VG 68 about 68 cSt, VG 100 about 100 cSt and VG 150 about 150 cSt. A higher number means a thicker oil. The number does not tell you the quality of the oil. Quality comes from the base oil, mineral or synthetic, and from the additives that resist oxidation and foam.
Screw or reciprocating: the grades at a glance
Rotary screw compressors: usually VG 46 or 68
In a screw compressor the oil is injected into the compression chamber. It seals the gap between the rotors, lubricates the rotors and bearings, and carries heat away. It must then separate cleanly from the compressed air. VG 46 is the common choice at normal temperatures. VG 68 is used where the compressor room or the oil runs hotter, or where the maker asks for it.
Reciprocating compressors: usually VG 100 or 150
In a piston compressor the oil lubricates the cylinders, rings and valves, and the discharge temperature is high. VG 100 is the usual grade for single-stage units. VG 150 is used for units with higher discharge temperatures, such as many two-stage compressors. The oil must resist oxidation and form little carbon, because carbon on the discharge valves is the main failure in these machines.
Vane compressors and vacuum pumps: follow the maker
Rotary vane compressors usually use VG 46 or 68 oil made for the purpose. Vacuum pumps need an oil made for vacuum duty. For both, use the grade and type the maker names.
How to choose: three steps
- Start with the compressor maker. The manual or the plate on the machine gives the grade and the type of oil. This is the first answer, and it overrides every rule of thumb.
- Match the oil to the compressor type. Screw and vane compressors usually run VG 46 or 68. Reciprocating compressors usually run VG 100 or 150. Do not use hydraulic oil or engine oil as a shortcut in a compressor.
- Look at how hot the machine runs. Hot oil gets thinner, so a compressor in a hot room or a long duty cycle may need the thicker grade of the two the maker allows. A cold start may need the thinner one.
Mineral or synthetic compressor oil?
Both are made in the same ISO VG grades. Synthetic oil resists heat better and usually lasts longer between changes, so it suits continuous duty and hot plants. Typical drain intervals are about 2,000 to 4,000 hours for mineral screw compressor oil and 6,000 to 8,000 hours for synthetic. For reciprocating compressors they are about 1,000 to 2,000 hours for mineral and 4,000 to 6,000 hours for synthetic. These are typical figures only. Follow the maker's interval and check the oil condition.
What goes wrong with the wrong grade
Oil too thin
The oil film is weaker. You may see more wear on rotors, bearings, rings and cylinders, and a rise in temperature.
Oil too thick
The compressor works harder to move the oil. You may see hard starts in cold weather, more heat and higher power use. In a screw compressor, oil that does not separate well can also be carried into the air line.
Quick summary
- Screw and vane compressors: usually ISO VG 46 or 68.
- Reciprocating compressors: usually ISO VG 100 or 150.
- If the manual gives a grade, use that one.
For a wider look at oil choice by compressor type, read our compressor oil guide for screw, reciprocating and vane compressors.
Lukeron compressor oils
Lukeron makes a mineral Screw Compressor Oil in ISO VG 46 and 68 for rotary screw and vane compressors, a Synthetic Screw Compressor Oil for extended drain and high temperature duty, and a mineral Reciprocating Air-Compressor Oil. All are available in 20 litre and 210 litre packs. Tell us your compressor type, maker and working temperature, and we will help you choose the oil and send a free sample to test. Follow the compressor maker's grade and drain interval.